This invention relates to the field of transmission and recording of color television signals.
Video tape recorders are available that can directly record television signals as they are normally broadcast. These recorders must have very high bandwidth capabilities to record such signals and are, therefore, very expensive. The common video recorders that are available on the market are less expensive and have much lower bandwidth capabilities. Since these low cost recorders cannot directly record television signals accurately, various types of processing are performed on the signals to reduce the bandwidth requirements. Standard television signals contain composite chrominance and luminance information broadcast during the active video portion of a conventional video line. FIG. 1 illustrates an NTSC composite color video signal. The active video portion is approximately 52.5 microseconds in duration with the entire video line occupying 63.5 microseconds. The remaining 11 microseconds is reserved for synchronizing pulses (H-SYNC), clamping, transition times, and a color reference signal (BURST). A typical prior art method of recording such a signal was to first separate the luminance and chrominance information and then record the separated signals on tape by different techniques, most commonly by frequency modulating the luminance component onto a carrier and amplitude modulating the chrominance signal onto a lower-frequency subcarrier. The signal containing the chrominance information is then added to the frequency modulated luminance carrier and the combined signal is recorded.
The above described method is subject to a number of problems including several sources of picture degradation such as a poor chrominance signal-to-noise ratio, gain and delay inequalities between chrominance and luminance and distortion such as differential phase and gain. These problems become amplified when several generations of copying occur.
Several methods have been introduced which attempt to alleviate some or all of the above sources of degradation. One involves arranging the chrominance and luminance information serially. A conventional composite video signal is reformatted serially into a new signal containing lines of serially arranged chrominance and luminance information. The chrominance and luminance components are compressed before they are arranged into the serial format. The compression step is necessary since the serially reformatted signal also has a line duration of 63.5 us and if the components are not compressed they cannot "fit" into this time period when they are arranged serially. FIG. 2 illustrates a typical signal where the luminance and chrominance data have been compressed and serially arranged. The luminance data has been compressed from its original 52.5 microseconds, the length of the active line portion, to 46 microseconds with the chrominance being further compressed to 11.5 microseconds. The chrominance data is usually time compressed more than the luminance data, typically by a small integer such as 2, 3 or 4. (Chrominance data usually consists of two color difference signals, here Cr and Cb, and is arranged on alternate lines with the corresponding luminance information Y).
This type of serially reformatted signal is similar to a Multiplexed Analog Component (MAC) signal which is proposed for satellite and cable television transmission. A MAC signal is a serially formatted signal with a luminance component that has been compressed from its original 52.5 microseconds to 35 microseconds and a chrominance component that is compressed to 17.5 microseconds. Although the use of MAC signals has overcome many of the problems inherent in the use of NTSC signals, several new problems have developed.
Television signals are transmitted along cable systems in a nominal 6 MHZ band. The (picture) carrier, by convention, must be 1.25 MHZ above the bottom end of the band; and the system response falls off above about 5.25 MHZ above the bottom of the band which leaves a usable 4 MHZ of bandwidth. Using 3:2 compression for luminance, this means that after decompression, the luminance signal will have a bandwidth of only 2/3 of 4.0 MHZ or 2.67 MHZ.
A similar problem is encountered in the use of consumer grade video recording equipment commonly sold on the market. As noted previously, a typical recorder has bandwidth limitations and the use of compressed signals causes problems similar to those encountered during cable transmission. U.S. Pat. No. 4,335,393 to Pearson, for example, discloses a method of recording a serially reformatted signal. Pearson compresses the luminance and chrominance data from its original duration in the active video region to form a signal similar to the one illustrated in FIG. 2. This compressed signal is then recorded. When Pearson's signal is recovered for playback there will have been a significant bandwidth loss due to the compression step. Several systems have been introduced which attempt to solve this problem.
U.S. Pat. No. 4,467,368 to Horstmann discloses a method of serially recording luminance and chrominance data. Horstmann time-expands the luminance signal before recording in order to avoid the bandwidth problems inherent in a system such as Pearson's. Since Horstmann has time expanded his signal, however, the signal will not fit within the 63.5 us limit and Horstmann must use two separate channels when recording (see FIG. 4). Each channel consists of a time expanded luminance signal and a compressed chrominance signal arranged serially. Since the recording is being performed in two channels, there is twice as much recording time per line and the signal can therefore be time expanded. The drawback to this method is that a much greater amount of recording space is required.
U.S. Pat. No. 3,781,463 to Van den Bussche is directed to a method of recording luminance and chrominance information serially in a single recording channel without using a compressed luminance signal. Since the luminance signal is not compressed, the chrominance signal must be compressed to a much greater degree than was necessary in other prior art systems to satisfy the time requirements. For example, the ratio between the length of the active line time and the length of the compressed chrominance signal is approximately 8 to 1. Such a high compression ratio means that there is also a high ratio between the time occupied by the luminance and chrominance components which will cause the signals to become degraded, with the color having the tendency to smear over the luminance. Such high ratios of time compression also reduce chroma Signal-to-Noise Ratio to unacceptable levels of performance. Rhodes, C. W. "A tutorial on Improved Systems for Color Television Transmissions by Satellite." IEEE Transactions on Broadcasting Vol BC-31 No. 1 March 1985.